1. Field of Invention
The present invention relates to a multi-phase voltage-control oscillator (VCO) structure. More particularly, the present invention relates to a structure with inductors between which an inductance effect is generated.
2. Description of Related Art
Currently, with the progress of communication industry, integrated circuits (IC) have been widely applied in wireless communication. In design, the ICs must meet several requirements such as low-voltage operation and low power consumption. In a wireless communication system, after the antenna receives a radio frequency (RF) signal, a frequency down converter constituted of an LNA, a VCO, and a mixer is used to down-convert the RF signal to an intermediate frequency signal. For the signal to be transmitted, a frequency up converter constituted of a VCO, a mixer, and a high-power amplifier is used to up-convert the intermediate frequency signal to the RF signal, and then the up-converted signal is transmitted via the antenna. A RF microwave circuit is located at the most front electrode of the entire communication system, and the characteristic of the RF microwave circuit directly affects the quality of communication, so the RF microwave circuit is a crucial part.
The VCO plays an important role in modern communication systems. As the frequency up- and down-conversion is mainly performed by the mixers at the front electrode of the VCO and the transmitter/receiver, the noise of the VCO will influence the noise level of the entire transceiver. Therefore, it is an important subject how to reduce the phase noise of the VCO.
Since the architecture adopted by the RF receiver is a distinct one, the requirements for the multi-phase signal become relatively strict. A quadrature-phase signal, for example, can be generated by the following methods.
(1) Combination of VCO, polyphase-filter, and output buffers. In this method, four output buffers are required. If the four output buffers are disposed between the filter and the VCO, the output buffer consumes a lot of power. If the output buffer between the filter and VCO is removed, the capacitance of a resonance cavity increases accordingly, resulting in large power consumption and the increase of the phase noise. In addition, in order to obtain a good match of the filter, the integration is the only choice. Therefore, the method of generating the quadrature-phase signal has a disadvantage of requiring large chip area.
(2) Frequency division. In this method, the required chip area is small. However, the structure must use a master-slave flip-flop which must operate at a frequency same as that of the VCO, i.e. twice of the operating frequency. Therefore, this method has a problem of large power consumption.
(3) Two cross-coupled VCOs. The power consumption of this method is much less than that of the above two forms. However, the method has a disadvantage that the use area is twice of a typical differential VCO area.
The current quadrature-phase VCO (QVCO) mostly uses a so-called LC tank as a basic oscillator architecture and an additional MOS device to generate a coupled signal. The method has lower phase noise, and the QVCO adopting the LC tank can be classified into two categories.
The first are those having the MOS device generating the coupled signal connected in parallel with the MOS device generating a negative resistance in the LC tank VCO architecture. Referring to FIG. 1, a circuit diagram of an LC tank QVCO is provided. The architecture is characterized in that the phase error of the quadrature-phase output is closely relevant to the MOS device generating the coupled signal. The bigger the MOS device generating the coupled signal is, the smaller the phase error of the quadrature-phase signal output by the QVCO is. However, the oversized coupled MOS device results in that the phase noise get worse due to the extra coupled MOS device, and also incurs an additional current consumption, which leads to a great increase of power consumption.
The second are those having the MOS device generating the coupled signal connected in series with the MOS device generating the negative resistance in the LC tank VCO architecture, in which there are two placement manners of the two MOS devices. Referring to FIG. 2, the MOS device generating the coupled signal can be placed at the source of the MOS device generating the negative resistance in the LC tank VCO architecture, which is called as the bottom-series QVCO. Referring to FIG. 3, the MOS device generating the coupled signal can also be placed at the drain of the MOS device generating the negative resistance in the LC tank VCO architecture, which is called as the top-series QVCO. The top-series QVCO has a better performance than the bottom-series QVCO.
Although the performance of the second series architecture is better than that of the parallel QVCO, as the stacked MOS devices are disposed in the path from the voltage supply electrode to ground electrode of the series architecture, the circuit cannot operates under low voltage supply.
Therefore, it is a problem to be solved how to operate the multi-phase VCO under low voltage and maintain good performance.